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Design >> Analog Design >> Output swing of VCO inverter chain
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Message started by adesign on Jan 15^th, 2007, 6:04am

Title: Output swing of VCO inverter chain
Post by adesign on Jan 15^th, 2007, 6:04am

Dear all,

I'm designing a ring-oscillator with 5 inverter stages(current starved inverters). Each inverter stage has two current sources. One is to charge the capacitor formed by the next stage and the other is to discharge that. Both the current sources have the same dc current level.(The basic schematic is attached herewith for one of the inverter stage).

For keeping both the current sources(top and bottom) in saturation, we need to limit the swing of the inverter chain. I'm trying to reduce that by reducing the length as well as reducing the current supplied by the sources. However, I've reached the limit of minimum length of the devices. I'm also seeing that whenever the output frequency is high then the swing is limited whereas when the output frequency is low the swing is very high(almost rail-to-rail).

Could you please suggest me any alternate way to reduce the swing or to keep the sources in saturation?

Best regards,

Title: Re: Output swing of VCO inverter chain
Post by jeffyan on Jan 15^th, 2007, 11:06pm

adesign wrote on Jan 15^th, 2007, 6:04am:

hi,
i think you can try to add a diode connection mos transistor onto the output. in this way,
the tail current can keep acting as a current source. this is an original idea, which is not suitable for your case.
but after some modification, you can realize a good clipping swing.
good luck
jeff

Title: Re: Output swing of VCO inverter chain
Post by topquark on Jan 16^th, 2007, 12:33am

Yes, the current sources could be derived from a mirror, both for the PMOS and the NMOS.
But, I don't see why you need to reduce the output swing which would only worsen your phase noise.
Any other design criteria you're after?

Regards,
Gau

Title: Re: Output swing of VCO inverter chain
Post by adesign on Jan 16^th, 2007, 1:05am

Hi,

The current sources are derived from a mirror itself. However, for the large swings, both the current-sources may go into linear. This is the only concern I have.

Please comment.

Regards,